KERRY O'BRIEN: Melbourne motorists are going to have to swallow their frustration for at least another month before the leaking Burnley tunnel is again fully operational.

Five litres a second have been pouring into the tunnel since Monday, baffling tollway operator Transurban's engineers as to both cause and solution.

In the meantime, the city has suffered enormous traffic congestion, angry motorists and a State Government surprisingly resistant to criticise the company behind the project.

The tunnel, which only opened in December, completes the $2.2 billion Citylink project -- a controversial privately funded tollway that was itself bedevilled by delays and criticism.

Josephine Cafagna reports.

KIM EDWARDS, MANAGING DIRECTOR, TRANSURBAN: There is a defect in the works.

No, we did not expect a defect in the works.

PETER BATCHELOR, VICTORIAN TRANSPORT MINISTER: A lot of money, a lot of thought and hard work has gone into delivering this project.

It should not have happened.

DR BILL BAMFORD, ASSOCIATE PROFESSOR OF ENGINEERING, MELBOURNE UNIVERSITY: In principle, no tunnel can be constructed without some minor quality control problems at the end of the project.

JOSEPHINE CAFAGNA: It was hailed as an engineering marvel, the last and most ambitious section of Melbourne's new Citylink tollway.

A three-lane tunnel under the Yarra River, dramatically cutting travel time from west to east.

Trouble is, it's been plagued with problems from the first sod -- delays technical problems with computer tolling systems, problems with the concrete floor leading to more construction delays, and now it's leaking water through cracks at the rate of 300 litres a minutes.

MAX LAY, INDEPENDENT REVIEWER: It's a different leak in quantity it's not small.

In quantity it's about average.

But it's a different sort of leak so that's why a bit of time's being spent on diagnosing what's caused this problem.

JOSEPHINE CAFAGNA: The Burnley tunnel has been closed since Monday.

It wasn't until late today we heard the reasons why.

MAN: So the toe has moved and has caused that crack pattern the wall.

WOMAN: But why has the toe moved?

MAN: Because there is a defect in the works.

JOSEPHINE CAFAGNA: The closure is understood to be costing Transurban $200,000 a day in lost toll revenue.